The present invention relates generally to an image processing apparatus, and more particularly to an image processing apparatus in which a halftone image data is generated by making use of a prescribed image processing technique.
Conventionally, the dither process is used for carrying out bilevel rendition of a multilevel tone data of an image, through halftoning, to a bilevel data which can be appropriately output to a printer. In a case of the dither process, pixels of an input image data are compared with a predetermined dither matrix pattern to produce a bilevel data of pixels of an output image, but there is a problem in that picture resolution being achieved by the dither process is not compatible with the quality of an output image with good visual tone data. There is another halftoning method, which is an improved halftoning technique that is aimed at realizing suitable picture resolution and compatible visual tone quality of the output image. Such a halftoning technique is called herein an error diffusion method. In a case of this error diffusion method, an error of an optical density or intensity of a pixel of an image being produced in a bilevel rendition process is stored in a memory for subsequent use in the bilevel rendition process of other pixels adjacent to the subject pixel being considered, so that the intensity of pixels of the image is maintained after the bilevel rendition process is carried out. In order for determining a gray level via the error diffusion method, the influences of neighboring pixels on the subject pixel and the results of data conversion are incorporated. In the error diffusion method, errors of dot density between dots of an original image and dots of a reproduced image are locally diffused so that an image having the least approximation to the original image is generated. The above mentioned error diffusion technique of this kind is disclosed, for example, in "Digital Halftoning" (the name of a transaction contained in a journal published by the Institute of Television Engineers of Japan).
However, if the above mentioned error diffusion process is applied to a printer for printing an output image in which only a bilevel data is assigned to each pixel of the output image, there is a problem in that undesired pseudo stripe or moire pattern appear at portions of the output image where the dot density of the image does not clearly change. Especially when a screened halftone image data is processed through the error diffusion process and output to the above mentioned printer, it is difficult for the printer to suitably output continuous tone image which is in accordance with the original screened halftone image. In addition, in a case of a multilevel tone printer which is capable of varying a gray level of pixels of an output image by controlling the exposure time of laser light, it is difficult to suitably control the gray level of the output image for each pixel. Therefore, there is a problem in that the intensity of the actual output image cannot be made suitably in accordance with the value of control data used for outputting the output image having multilevel tone data to the multilevel tone printer.